Method of preparing porous calcium phosphate granules

ABSTRACT

The invention describes a method of preparing macroporous calcium phosphate granules with NaCl porogen technique, wherein a calcium phosphate self-setting cement powder is mixed with 30 to 80 wt % NaCl. The calcium phosphate granules comprise macro- and micropores communicating with one another substantially throughout the body with a porosity of 40% or more and having the same mineral as those of human bones. Said granules are used as a substitute or repairing material for bone, carrier material for drug delivery and gradual release system.

The invention relates to a simple method of preparing porous calciumphosphate granules with the NaCl porogen technique.

Insertion (or presence) of macropores (pores>100 micron in size ornests, which allow sites for living cell attachments) into calciumphosphate bone cements increase their resorption time (while the boneremodelling around the implant is taking place within the first fewmonths) upon their implantation into the body. In that sense, makingeasily applicable (by the surgeon) bone cement materials with asignificant amount of macropores in them is quite a hot topic in thefield of bioceramics (ceramics which are designed, synthesized and usedin biological and clinical applications).

Hong-Ru Lin et al. (J. Biomed. Mater Res. (Appl. Biomater.) 63: 271-279,2002) describes a mixture of NH₄HCO₃ and NaCl particles used as aporogen additive to fabricate highly macroporous biodegradablepoly(lactic-co-glycolic acid) scaffolds. A two-step salt-leachingprocess was performed after the sample had become semi-solidified. Adisadvantage of this method, when its application for calciumphosphate-based cement compositions considered, is the fact that the useof basic salts such as ammonium bicarbonate destroys the deliberatelyadjusted chemical composition of the cements, especially during thefollowing washing steps.

There are several other (porogen) techniques which employ the initialmixing of calcium phosphate ceramics together with organic (or eveninorganic) materials (porogens), followed by heating them atsufficiently high temperatures (100° to 1400° C.) to volatilize theporogen components, and forming gas bubbles in their places, which thenprovide the desired porosity to the product.

Another decent porogen technique for preparing porous calcium phosphategranules is using “ice crystals” as the porogen (instead of NaCl or anyother thing), but on a larger scale production, the precise control ofthe size and morphology of those ice crystals (without significantlysacrificing the mechanical strength/stability of the final granules)requires quite a high expenditure in terms of equipment and coldenvironments to be acquired/set at the production site.

An object of the present invention is to provide a simple method forpreparing macroporous calcium phosphate granules via NaCl porogentechnique, which avoids the above-mentioned disadvantages from the priorart.

Upon further study of the specification and appended claims, furtherobjects and advantages of this invention will become apparent to thoseskilled in the art.

These objects are achieved by a method of preparing macroporous calciumphosphate granules characterized in that the method comprising the stepsof:

-   a) mixing a calcium phosphate self-setting cement powder and 30 to    80 wt % sodium chloride-   b) wetting the powder with a mixture of ethanol and Na₂HPO₄ aqueous    solution as specific setting solution of the cement.-   c) kneading the wet powder body to form a cake-   d) sieving the wet cake in an automatic sieving machine (with    multiple sieves) to in situ form granules of desired sizes-   e) leaching out of the sodium chloride (porogen) with water-   f) placing the granules in dilute Na₂HPO₄ solution-   g) washing the granules with water-   h) drying the obtained granules.

The porous calcium phosphate granules (size range 0.5 to 6 mm) are usedas a substitute, or repairing material for bone, carrier material fordrug delivery and gradual release system.

The present porous granules of a self-setting calcium phosphate cementare produced by preparing a powder mixture of the said calcium phosphatecement with sodium chloride (preferred is a weight ratio of NaCl powderto cement powder=1.7 to 1.8, more preferred 1.75), wetting and kneadingit first with a mixture of its setting accelerator solution (preferablywith a Liquid (in ml)-to-cement powder (in grams) ratio of 0.30) and asmaller amount of ethanol, and granulating the paste in an automaticsieving machine with multiple sieves of desired granule sizes, beforethe cement setting to take place in the first 8 minutes of mixing withthe above solution, followed by first dissolving out the sodium chlorideat room temperature in pure water, second rinsing the formed granules ina dilute hydrochloric acid solution to create interconnectivity amongthe pores of the granules, and third immersing the granules in thecement's own setting solution at the human body temperature to increasetheir mechanical strength, finally followed by drying and sieving of theformed granules.

It is advantageous using NaCl, because human plasma and human bodyfluids contain 5.8 g of NaCl per liter of them. Therefore, NaCl is aninorganic material, which is perfectly compatible with the human plasma.

A further advantage of the present method is that it does not employ anyhigh temperature treatment (to form porosity), which otherwise couldeasily destroy the precise balance (amount and composition-wise) to beretained between the calcium phosphate constituents.

A further advantage of the said method is that it leaves behind itsfootprint. NaCl crystals are cubic due to their crystallographic nature.Moreover, NaCl crystals do exhibit dislocation steps or kinks on theirsurfaces (readily visible with scanning electron microscopes) and thepores left behind (after dissolving out those NaCl crystals) in thecalcium phosphate cement matrix are like the “replicas” of thosecrystals, and the product can then easily be identified (as depicted inthe FIGURE) according to its manufacturing process.

Advantageous is also that the time of processing (i.e. the time calciumphosphate powder and NaCl powder are in contact with one another as anintimate mixture) is extremely short, within the following 20 minutes,in the washing step, NaCl is taken out of the system. Moreover, themethod is, in terms of the manufacturing costs on a larger scale, cheap.

The present method can be used with any “self-setting calcium phosphatecement” already available. Suitable cements, which can be used, are:powder mixtures of calcium phosphate “species” (one or more of them tobe present in the final powder body) or “phases” or “constituents”

-   -   a) Amorphous calcium phosphate (ACP)    -   b) MCPM (monocalcium phosphate monohydrate: Ca(H₂PO₄).H₂O)    -   c) TTCP (tetra calcium phosphate: Ca₄(PO₄)₂O)    -   d) alpha-TCP (tricalcium phosphate: Ca₃(PO₄)₂)    -   e) beta-TCP (Ca₃(PO₄)₂)    -   f) DCPD (dicalcium phosphate dihydrate: CaHPO₄.2H₂O)    -   g) DCPA (dicalcium phosphate anhydrous: CaHPO₄)    -   h) HA (calcium hydroxyapatite: Ca₁₀(PO₄)₆(OH)₂)    -   i) Calcium carbonate (CaCO₃)    -   j) Calcium hydroxide (Ca(OH)₂) and (as minor components in a        calcium phosphate matrix)    -   k) biocompatible polymers (such as polylactic acid, PLA or        polylacticglycolic acid, PLGA or polymethylmetacrylate, PMMA)    -   l) biocompatible silicates (such as silicate compounds to be        formed out of the quaternary system of Na₂O—P₂O₅—CaO—SiO₂)        prepared/mixed (with one or more of the above components) with a        variable Ca/P over the range of 1.0 to 2.0, more preferably over        the range of 1.2 to 1.55, when mixed with a proper setting        solution (ranges from pure water to dilute phosphoric or citric        acid solutions and even to alkali phosphate dilute aqueous        solutions, i.e., the setting solutions can either be acidic,        neutral or basic with respect to their pH values), sets in a        relatively short time at the human body temperature to gain a        significant strength (variable in the range of 5 MPa to 100        MPa).

Preference is furthermore given to a biocement (as calcium phosphateself-setting cement powder) of 50 to 70% α-TCP, 2 to 9% HA, 20 to 30%DCPA and 5 to 10% CaCO₃.

The calcium phosphate granules have pore sizes in the range of 0.5 to 6mm, preferably about 2 to 3 mm.

The porosity of the present granules is 30 to 70%, preferably about 40to 50%.

Preference is furthermore given to the calcium phosphate cement powder,which is mixed with 60 to 65 wt % sodium chloride powder.

It is further preferred that 60 to 70 wt % of sodium chloride powderhaving particle sizes less than 0.25 mm and the remaining 30 to 40 wt %having particle sizes greater than 0.25 mm.

The macropores of the calcium phosphate granules have middle diametersof 50 to 1000 microns, preferably about 50 to 400 microns.

The invention is described in detail below in terms of the followingworking example.

In the foregoing and in the following examples, all temperatures are setforth uncorrected in degrees Celsius; and, unless otherwise indicated,all parts and percentages are by weight.

The FIGURE shows an electron microscope photograph of the formedgranules.

WORKING EXAMPLE Preparation of the Macroporous Calcium PhosphateGranules

To prepare the macroporous calcium phosphate granules, Calcibon® powderand NaCl powder in a molar ratio of 1:6.2, corresponding to 40 g: 70 gare mixed in dry form in a plastic box in a Turbula mixer for 90 minutes(without grinding balls in the box). 60 wt % of NaCl powder used abovehave particle sizes less than 0.25 mm, and the remaining 40 wt % of itwith particle sizes greater than 0.25 mm. The powder mixture is thenwetted and thoroughly kneaded with a premixed solution of 6.7 ml ethanolplus 12 ml aliquot of 3.5 wt % Na₂HPO₄ solution in a bowl-like containerfor 3 to 4 minutes. The wetted cement+NaCl cake is then immediatelyplaced on an automatic sieving machine which has the sieves in desiredmesh opening sizes to exactly yield the desired granule sizes. Sievingis performed and completed in 2 minutes. Humid granules of desired sizesare thus produced in situ on the selected sieves. Granules of differentsizes are left to dry in the ambient atmosphere and temperature forabout 1 hour. Next, the porogen (NaCl) is leached out with water (watermust be replenished continuously) at room temperature (RT) (totalresidence time of the granules in the washing water must at least be 72hours). The washed granules are dried at 50-60° C. for 24 hours andplaced in 1 wt % Na₂HPO₄ solution at 37° C. (ratio of “weight ofgranules (in gram)” to “liquid volume (in ml)” must be 0.04 (to increasethe mechanical strength of the granules), followed by washing with waterand drying at 50-60° C.

EDXS analysis performed on the final, dried granules showed that theydid not contain any Na and Cl ions originating from the use of NaCl,after washing with water.

The formed granules have a density of 1.6 g/cm³, and they have a waterabsorption percentage of 150.

1. A method of preparing macroporous calcium phosphate granulescomprising: a) mixing a calcium phosphate self-setting cement powder and30 to 80 wt % sodium chloride, b) wetting the mixture from a) withethanol containing Na₂HPO₄ solution, c) kneading the mixture from b), d)in situ forming of humid granules on a sieve machine, e) leaching outthe sodium chloride with water, f) placing the granules in Na₂HPO₄solution, g) washing the granules with water and h) drying the obtainedgranules.
 2. The method according to claim 1 wherein said calciumphosphate powder is a biocement of 50 to 70% α-TCP, 2 to 9%hydroxylapatite, 20 to 30% dicalcium phosphate anhydrous and 5 to 10%CaCO₃.
 3. The method according to claim 1 wherein the macroporouscalcium phosphate granules have pore sizes in the range of 0.5 to 6 mm.4. The method according to claim 1 wherein the macroporous calciumphosphate granules have pore sizes in the range of 2 to 3 mm.
 5. Themethod according to claim 1 wherein the porosity of the granules is 30to 70%.
 6. The method according to claim 1 wherein the porosity of thegranules is 40 to 50%.
 7. The method according to claim 1 wherein thecalcium phosphate powder is mixed with 60 to 65 wt % sodium chloridepowder.
 8. The method according to claim 1 wherein 60 to 70 wt % ofsodium chloride powder having particle sizes less than 0.25 mm and theremaining 30 to 40 wt % having particle sizes greater than 0.25 mm. 9.The method according to claim 1 wherein the macropores of the granuleshave middle diameters of 50 to 1000 microns.
 10. The method according toclaim 1 wherein the macropores of the granules have middle diameters of50 to 400 microns.
 11. A method according to claim 1, wherein the weightratio of sodium chloride to calcium phosphate self-setting cement powderis from 1.7 to 1.8.
 12. A method according to claim 11, wherein theweight ratio of sodium chloride to calcium phosphate self-setting cementpowder is about 1.75.